Data Transmission and Reception
11-46
When the transmitter is taken out of reset (XRST = 1), it is in a transmit ready
(XRDY = 1) and transmit empty (XEMPTY = 0) condition. If DXR is loaded by
the CPU or the DMA controller before FSX goes active, a valid DXR-to-XSR
transfer occurs. This allows for the first element of the first frame to be valid
even before the transmit frame sync pulse is generated or detected. Alterna-
tively, if a transmit frame sync is detected before DXR is loaded, 0s are output
on DX.
Figure 11–29 depicts a transmit underflow condition. After B is transmitted, B
is retransmitted on DX if you fail to reload the DXR before the subsequent
frame synchronization. Figure 11–30 shows the case of writing to DXR just be-
fore a transmit underflow condition that would otherwise occur. After B is trans-
mitted, C is written to DXR before the next transmit frame sync pulse occurs
so that C is successfully transmitted on DX, averting a transmit empty condi-
tion.
Figure 11–29.
Transmit Empty
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
B4
B5
XEMPTY
B6
B7
B0
B1
B2
B3
B4
B5
B6
A1
XRDY
DX
FSX
B7
A0
CLKX
Write of DXR (C)
DXR-to-XSR copy (B)
Figure 11–30.
Transmit Empty Avoided
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
XEMPTY
XRDY
DX
FSX
CLKX
DXR-to-XSR copy (B)
Write of DXR (C)
DXR-to-XSR copy (C)
C5
C6
C7
B0
B1
B2
B3
B4
B5
B6
A1
B7
A0